The invention relates to a catalytically active particulate filter which is suitable especially for the removal of particulates from the exhaust gas of internal combustion engines operated predominantly with a stoichiometric air/fuel mixture. The invention also provides a process for removing particulates, carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas of internal combustion engines operated with a predominantly stoichiometric air/fuel mixture, and a device suitable for this purpose.
Exhaust gases from internal combustion engines operated with a predominantly stoichiometric air/fuel mixture are cleaned in conventional methods with the aid of three-way catalytic converters. These are capable of converting the three essentially gaseous pollutants of the engine, specifically hydrocarbons, carbon monoxide and nitrogen oxides, simultaneously to harmless components. Apart from the gaseous hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx) pollutants, the exhaust gas of gasoline engines also contains ultrafine particulate matter (PM), which results from the incomplete combustion of the fuel and consists essentially of soot.
In contrast to the particulate emission from diesel engines, the particulates in the exhaust gas of predominantly stoichiometrically operated gasoline engines are very small, i.e. they have an average particle size less than 1 μm. Typical particle sizes are in the range from 10 to 100 nm. Owing to the endangerment potential emanating from respirable ultrasmall particulates (“fine dust”), the introduction of the EU-5 exhaust gas standard in Europe from 2010 will impose a limit to the permitted particulate emission for gasoline engines too. Owing to the switchover in the particulate measurement method from methods which detect the particulate mass to particulate counting methods, a changeover in the limit from the particulate mass limit to a more critical particulate number limit can be expected for the subsequent European exhaust gas standard EU-6. Accordingly, future exhaust gas cleaning concepts for stoichiometrically operated internal combustion engines will also have to include effective devices for removing particulates.
Methods and devices for removing particulates from the exhaust gas of diesel engines are well known. These include catalytically active diesel particulate filters, catalytic coatings on particulate filters for diesel exhaust gas cleaning methods usually being oxidation catalysis coatings which serve in particular to facilitate the combustion of the soot particulates to be deposited on the filter and thus lower the soot ignition temperature. The lowering of the soot ignition temperature lowers the “balance point temperature” (BPT) which is characteristic of the filter. “Balance point temperature” (BPT) refers to the temperature at which the amount of particulates accumulated in the filter per unit time corresponds to the amount of particulates burnt off per unit time. The BPT is a parameter which is characteristic of the catalytically activated filter and depends on the selected operating point of the engine and on the exhaust gas temperature, the exhaust gas mass flow and the composition of the exhaust gas on entry to the filter.
WO 00/29726 describes a device for cleaning diesel exhaust gases in which a catalytically active filter is present. The filter comprises a first catalyst which contains a first platinum group metal and a first cerium compound. The device also contains a second catalyst which contains a second cerium compound. Embodiments are described in which both catalysts are arranged on the filter substrate. The system is notable in that, with the aid of the cerium compound present in the second catalyst, the volatile organic fraction (VOF) adhering to the soot particulates in the diesel particulate mass can be removed by oxidation. Therefore, in the particularly preferred embodiments, the second catalyst is arranged upstream of the catalytically active diesel particulate filter.
WO 02/26379 to this applicant describes a process for reducing the level of carbon monoxide, hydrocarbons and soot particulates in the exhaust gas of lean burn engines using a particulate filter, wherein the soot particulates have a soot ignition temperature Tz and the particulate filter is regenerated from time to time by raising the temperature of the particulate filter above the soot ignition temperature and combusting the soot particulates. The particulate filter used is provided with a catalytically active coating which, to reduce the ignition temperature of the soot, comprises at least one oxygen-storing component and at least one of the platinum group metals platinum, palladium and rhodium. In a preferred embodiment of the particulate filter, the coating contains a second group of compounds which serves to oxidize carbon monoxide and hydrocarbons and contains at least one of the platinum group metals platinum, palladium and rhodium supported on a support material selected from a group consisting of alumina, silica, titania, zirconia, zeolite and mixtures thereof. The two substance groups are preferably arranged in two separate layers one on top of the other, the coating which is active in terms of oxidation catalysis being applied directly to the inflow channels of the filter substrate and the coating which reduces the soot ignition temperature being arranged directly thereon, such that the exhaust gas to be cleaned first comes into contact with the coating which lowers the soot ignition temperature.
The lowering of the soot ignition temperature is of particular significance in the case of use of particulate filters to clean diesel exhaust gases, since, owing to the “cold” combustion profile in the diesel engine, the implementation of temperatures above 400° C. in the downstream exhaust gas system is frequently difficult. Accordingly, there is a number of further patent applications which have particulate filters with a coating which is active in terms of oxidation catalysis and/or a coating which lowers the soot ignition temperature for their subject matter. Some applications, for example US 2006/0057046 A1, additionally take account of the exhaust gas backpressure problems of filter substrates. In this context, special three-dimensional arrangements of the catalytic coating are used to obtain very substantially homogeneous flow of the exhaust gas through the filter walls over the entire length of the component.
In the removal of particulates from the exhaust gas of predominantly stoichiometrically operated gasoline engines, there are significant differences with regard to the exhaust gas temperature, the exhaust gas composition and the nature of the particulates. These have to be reflected in a suitable exhaust gas cleaning concept.
As already described, particulates in the emissions of a gasoline engine have a much smaller average particulate size. This leads to the effect that not all filter bodies known in the prior art are suitable for cleaning the exhaust gas to very substantially eliminate particulate emissions, and it is instead necessary to use substrates which are not permeable even to ultrafine particulates. Accordingly, US 2006/0133969 A1 describes an exhaust gas system for internal combustion engines which comprises a catalyzed ceramic wall flow filter provided with a washcoat composition which comprises an oxidation catalyst and a nitrogen oxide adsorber, wherein the uncoated ceramic wall flow filter has a porosity of >40% and an average pore size of 8-25 μm, and the washcoat composition has a D50 less than/equal to 8 μm. The washcoat composition comprises platinum and/or palladium as an active component in terms of oxidation catalysis, and a metal oxide of a metal selected from the group of the alkali metals, the alkaline earth metals and the rare earth metals or mixtures thereof as the nitrogen oxide adsorber. In one embodiment, rhodium is applied to the outlet channels of the filter. The catalytically activated filter described in US 2006/0133969 is an “NOx particulate trap” (NPT), which is suitable especially for removing particulates and nitrogen oxides from the exhaust gas of predominantly lean burn diesel and gasoline engines and should be operated cyclically with cycles of rich and lean exhaust gas.
Gasoline engines which are operated with a predominantly stoichiometric air/fuel mixture usually have significantly higher exhaust gas temperatures than lean burn engines. Therefore, a catalytically coated particulate filter which is used to clean exhaust gases of predominantly stoichiometrically operated internal combustion engines must in particular feature a high thermal aging stability. More particularly, such a particulate trap must withstand temperatures up to 1100° C. even over a prolonged period and significant temperature changes with sufficient catalytic activity. The catalytically activated diesel particulate filters known from the prior art generally do not satisfy these requirements.
EP 1 300 193 A1 to this applicant describes a process for catalytic conversion of pollutants in the exhaust gas of internal combustion engines, wherein the exhaust gas passes through a porous support body wall which has an open pore structure and is optionally coated catalytically on both sides. The support body itself may consist of catalytically active material. A particular embodiment of the process is suitable for cleaning the exhaust gases of stoichiometrically operated internal combustion engines. In this case, a substrate is provided with a three-way catalytic converter coating which can simultaneously convert nitrogen oxides, hydrocarbons and carbon monoxide.